The salinity of ocean waters is calculated as the amount of salt (in gm) dissolved in _____ gm of seawater.

The correct answer is D, 35‰. Every kilogram of sea water carries about 35 grams of dissolved salts, written as 35 parts per thousand, and roughly 27 of those grams are common salt, sodium chloride. Salinity is not the same everywhere. It falls where big rivers pour in fresh water or where heavy rain and melting ice dilute the sea, and it rises in hot, enclosed basins where evaporation is strong. That is why the Red Sea stands near 41 parts per thousand while the Baltic Sea is only about 7. A is wrong because 130 parts per thousand is far beyond any open ocean and belongs to a salt pan. B is wrong because 25 parts per thousand describes a brackish coastal patch near a river mouth, not the ocean as a whole. C is wrong because 225 parts per thousand is a laboratory strength brine. Exam tip: 35 average, Red Sea the saltiest sea, Baltic the least salty.

The correct answer is D, Sheet erosion. Sheet erosion peels a thin, even film of topsoil off a whole slope, leaving no channel or scar behind, so a farmer may not notice it for years while fertility quietly drains away. It begins with raindrops splashing the bare surface loose, after which a shallow sheet of run-off carries the finest and most fertile particles off the field. The only hints are a paler soil colour, exposed pebbles and roots standing above the ground. A is wrong because gully erosion cuts deep, wide channels that nobody can miss and that often put land out of cultivation. B is wrong because stream bank erosion eats sideways into a river bank and shows up as caving earth. C is wrong because rill erosion carves small finger-like grooves that are shallow but still plainly visible. Exam tip: order them by how easily they are seen — sheet, then rill, then gully.

The correct answer is D, Tundra. In Koppen’s classification the group of cold climates, marked E, holds two types, the tundra climate and the ice cap climate. The tundra belt lies mainly north of the Arctic Circle along the northern edges of North America, Europe and Asia. Its summers are short and cool, the average temperature of the warmest month stays below ten degrees Celsius, the subsoil stays frozen as permafrost, and the plant cover is limited to mosses, lichens and low shrubs, with no trees. A is wrong because the humid subtropical type belongs to the warm temperate group, which covers northern India, southern China and the south eastern United States. C is wrong because the humid continental type falls in the cold snow forest group, one step milder than the polar group. B is wrong because highland climate is kept as a separate group, where conditions change with altitude rather than with latitude. Exam tip: Koppen’s groups are A tropical, B dry, C warm temperate, D cold snow forest, E cold or polar and H highland.

The correct answer is A, Vijayawada. Vijayawada in Andhra Pradesh has a large coal based thermal power station on the Krishna river, named after Dr Narla Tata Rao, but no nuclear plant. India’s nuclear stations are run by the Nuclear Power Corporation of India Limited, and the other three names in the question are all on that list. B is wrong because Narora, in Bulandshahr district of Uttar Pradesh on the bank of the Ganga, has a station with two pressurised heavy water reactors. C is wrong because Tarapur in Palghar district of Maharashtra is the country’s oldest nuclear station, commissioned in 1969 with boiling water reactors built with American help. D is wrong because Rawatbhata in Rajasthan stands on the Chambal near the Rana Pratap Sagar dam and carries the largest number of units of any Indian nuclear station. Kakrapar in Gujarat, Kaiga in Karnataka, and Kalpakkam and Kudankulam in Tamil Nadu complete the set. Exam tip: Tarapur was India’s first nuclear power station; Vijayawada is thermal, not nuclear.

The correct answer is B, West Bengal. India's first jute mill was set up at Rishra, on the bank of the Hooghly near Serampore in Bengal, in 1855 by George Auckland. The industry stayed along that river because the Hooghly belt had everything it needed together: jute grown in the deltaic plains of Bengal, cheap water transport, coal from the Raniganj field, labour from Bihar and Odisha, and the port of Kolkata to ship sacking and hessian out. Partition in 1947 left most of the jute fields in East Pakistan while the mills stayed in India, which hurt the industry badly. West Bengal still has the largest number of jute mills and the highest output. A is wrong because Odisha has only a few mills. C is wrong because Assam grows jute but its mills came later. D is wrong because Bihar's mills, at Katihar and elsewhere, also came after Bengal's. Exam tip: first jute mill at Rishra on the Hooghly in 1855; West Bengal leads in jute.